The present invention relates to arc lamps used to heat semiconductor wafers in semiconductor processing.
Advances in processing of integrated circuit devices, in particular, devices of decreased dimensions, have necessitated the development of new processing and manufacturing techniques. One of the strictest requirements imposed by the tight tolerances on the dimensions of such devices has been the need to restrict the amount of time an integrated circuit device is subjected to high temperatures. This has lead to the development of a rapid thermal process step, whereby the semiconductor device is irradiated with an optical or incandescent source powerful enough to quickly raise the temperature of the semiconductor device to the desired process temperature and hold it at that temperature for a sufficient time to accomplish a specific process step.
Early demonstrations of rapid thermal processing were accomplished with lasers. From a manufacturing standpoint, however, lasers have the disadvantage of high cost, low efficiency, small process areas and the need to raster or step the laser beam across the work surface with the result being a nonuniform step coverage. Since the rapid thermal heating process step does not require coherent radiation, other practitioners of the art have used incandescent heat sources, either glowing carbon strips or tungsten-halogen lamps. These heat sources have the disadvantage that their spectral output is a function of the source temperature. As more power is applied to the lamp (or carbon strip) to raise the temperature of the semiconductor device, the spectral content of the radiation changes and, therefore, the amount of energy absorbed in the semiconductor is a non-linear function of the power. In addition, at low input power, most of the energy from the incandescent lamps and carbon strips is at long wavelengths where semiconductors such as silicon or gallium arsenide appear transparent and can absorb little of the radiation.
One method used for heating semiconductors which allows low power operation of a continuous wave arc lamp is shown in U.S. Pat. No. 4,356,384 to Gat. The Gat apparatus moves the arc lamp across the semiconductor wafer so that only a small portion of the wafer is exposed to the light at any one time, thereby eliminating the need for the entire wafer to be illuminated at once and thus allowing a lower power lamp to be used. The technique also requires the use of a preheater to prevent damage to the semiconductor resulting from the large thermal gradients induced by the moving source.
U.S. Pat. No. 4,097,762 to Hilton shows a particular electrode composition wherein the arc discharge is obtained without heating the electrode. The electrode uses oxides of barium, calcium and aluminum.
U.S. Pat. No. 4,184,065 to Nagashima shows an ellipsoidal reflecting mirror for aiding in the heating of a wafer. By using the mirror to direct light already emitted from the lamp back onto the wafer, additional illumination and heating of the wafer is obtained without requiring more power to be applied to the lamp.